This invention relates to cast iron, particularly of the nodular graphite iron and compacted graphite iron types. More specifically, the invention concerns a process for making cast iron of nodular graphite iron and compacted graphite iron types.
Cast iron usually contains in excess of 2.5% carbon, and upon cooling the cast iron forms either gray iron or white iron. When the cast iron is cooled rapidly so that the carbon is retained in a martensitic matrix, white iron is formed. If the cast iron is not cooled rapidly to produce white iron, then flaked graphite, compacted (vermicular) graphite, or nodular graphite is formed, depending upon the composition and physical conditions of the cast iron.
Historically, ingot molds and stools have been cast from flaked graphite iron and more recently compacted graphite iron since these forms of cast iron have good thermal conductivity and thermal shock resistance values. Unfortunately, flaked graphite and compacted graphite iron molds tend to be brittle. Consequently, ingot molds and stools cast from flaked graphite or compacted graphite iron generally fail due to cracking.
Compacted graphite iron and nodular graphite iron are produced by alloying the molten iron with magnesium and other alloying elements. The magnesium addition is usually made just prior to the pouring of the cast iron into a ladle or mold so as to avoid fade by the reaction of the magnesium with the sulfur present in the molten iron or atmospheric oxygen in contact with the molten iron bath. If the magnesium reacts with sulfur or oxygen, the concentration of magnesium present within the molten iron decreases and varies in the time interval between the magnesium addition and the eventual casting of the molten iron. When the magnesium concentration fades, the formation of a particular type of graphite is difficult to predict since typically nodular graphite iron and compacted graphite iron form within specific magnesium concentration ranges. The difficulty in predicting the particular cast iron being formed results in numerous commercial disadvantages since one must wait until the cast iron is solidified before knowing its type. If nodular iron is to be produced, a manufacturer will ordinarily overtreat with magnesium to a level where it can be predicted with confidence that nodular iron will be formed.
Conventional processes for adding magnesium as an alloy to molten iron result also in the presence of undesired amounts of silicon or nickel in the molten iron. Magnesium is often added with a buffer, such as lime or carbon, to the molten iron. An example of a process which adds magnesium to molten iron is U.S. Pat. No. 4,036,641 in the name of Edward R. Evans et al in which an alloy containing silicon, magnesium, titanium and a rare earth metal is added to the iron in a single treatment. Titanium and rare earth metal additions broaden the range for producing compacted cast iron because of the inability to control magnesium accurately.
U.S. Pat. No. 4,236,944 in the name of Melib Yaman et al discloses a cast iron especially suited for ingot molds, but fails to show a nodular graphite iron ingot mold having a microstructure of substantially nodular graphite with an alloy composition of less than 0.005% by weight sulfur, at least 0.015% by weight magnesium and a silicon to manganese ratio of less than 1.5.
U.S. Pat. No. 3,880,411 in the name of Natalya A. Voranova et al discloses an evaporation bell which allows magnesium vaporization so as to prevent plugging with a minimum gas flow. This patent does not disclose the use of a protective slag cover to absorb sulfur and to prevent magnesium from reacting with atmospheric oxygen.
In brief, prior art processes for producing cast iron have numerous deficiencies which include magnesium fading due to the reaction of the magnesium with oxygen; the need to cast the molten iron immediately after the addition of magnesium to avoid magnesium fading; unpredictability of the type of cast iron produced; the presence of contaminants within the cast iron due to the addition of magnesium alloys and buffers; and inability to combine good heat conductivity and high erosion wear resistance in cast iron molds or stools.